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Message-ID: <b100a3c4-e5c3-41da-8c02-3a4986b49eec@linux.intel.com>
Date: Thu, 23 May 2024 21:44:33 +0800
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Cc: baolu.lu@linux.intel.com, iommu@lists.linux.dev,
 linux-kernel@vger.kernel.org, David Woodhouse <dwmw2@infradead.org>,
 Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
 Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in
 intel_pasid_get_entry()
To: Uros Bizjak <ubizjak@gmail.com>
References: <20240522082729.971123-1-ubizjak@gmail.com>
 <20240522082729.971123-2-ubizjak@gmail.com>
 <c1892ba9-4eae-40fc-b3ab-73d0f82a74ea@linux.intel.com>
 <CAFULd4Z=YkV1Hbs4DikPBwO-6rg8tfDLGeacSCnfbC02E5y+Cg@mail.gmail.com>
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From: Baolu Lu <baolu.lu@linux.intel.com>
In-Reply-To: <CAFULd4Z=YkV1Hbs4DikPBwO-6rg8tfDLGeacSCnfbC02E5y+Cg@mail.gmail.com>
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On 2024/5/23 21:34, Uros Bizjak wrote:
>>> +             if (!try_cmpxchg64(&dir[dir_index].val, &tmp,
>>> +                                (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
>> Above change will cause a dead loop during boot. It should be
> No, it is correct as written:
> 
> if (cmpxchg64(*ptr, 0, new))
> 
> can be written as:
> 
> if (cmpxchg64(*ptr, 0, new) != 0)
> 
> this is equivalent to:
> 
> tmp = 0ULL;
> if (!try_cmpxchg64(*ptr, &tmp, new))

The return value of both cmpxchg64() and try_cmpxchg64() is the old
value that was loaded from the memory location, right?

If so,

	if (cmpxchg64(*ptr, 0, new) != 0)

is not equivalent to

	if (!try_cmpxchg64(*ptr, &tmp, new))

Best regards,
baolu